Pulping processes are employed to separate lignins from wood to produce a pulp. A solution containing sodium hydroxide is employed to dissolve lignins from the wood to yield a black liquor containing sodium hydroxide and depolymerized lignins. A recovery boiler is used to recover the sodium hydroxide and other compounds for reuse and release of the energy contained in the lignins to provide steam.
A state of the art Kraft pulping digestion process employs a solution containing sodium hydroxide and sodium sulfide to create a black liquor. After concentration of the black liquor, by means of a multiple-effect evaporator, the black liquor is sent to a recovery boiler to produce steam and a molten smelt containing sodium carbonate. The smelt of melted sodium compounds flows to a vat where it dissolves into green liquor The green liquor is slaked by calcium oxide to convert the sodium carbonate to furnish sodium hydroxide The calcium oxide, as a result, is converted to calcium carbonate which settles at the bottom of a white liquor clarifier tank. The resulting calcium carbonate, after separation of adhering white liquor, is transported to a lime kiln where the calcium carbonate is converted to calcium oxide. Thus the black liquor recovery cycle is completed.
Another pulping process employs a solution derived from sulfur burned to produce sulfur dioxide. The sulfur dioxide forms a bisulfite from a base, often magnesium, to form a solution. The solution containing bisulfites is used to depolymerize, disperse and dissolve lignins and create a xe2x80x9csulfite waste liquor.xe2x80x9d In the process, soluble lignin sulfonic acid salts are formed from the solution. Recovery of the xe2x80x9csulfite waste liquorxe2x80x9d is performed with a recovery boiler to produce sulfur dioxide and the inorganic base. Accordingly the recovery cycle has been completed
Thus, it is believed that only elaborate recovery methods have yet been developed.
This invention relates to a method of recycling a solution containing sodium hydroxide without using a recovery boiler, a lime kiln or a smelt.
Therefore, an object of this invention is to obviate many of the limitations and disadvantages of the prior art to depolymerize lignins from a solution containing sodium hydroxide.
Another object of this invention is to produce depolymerized lignins significantly free of sodium compounds.
Still another object of this invention is to produce depolymerized lignins significantly free of sodium compounds for use as a fuel.
An additional object of this invention is to provide recycling of a solution containing sodium hydroxide used to produce depolymerized lignins.
An significant object of this invention is to utilize a multiple-effect evaporator, previously required to concentrate black liquor, to remove water from the solution containing sodium hydroxide.
One more object of this invention is to separate black liquor from a soda pulping digestion process to produce depolymerized lignins and a solution containing sodium hydroxide
With the above and other objects in view, this invention relates to the novel features and alternatives and combinations presently described in the brief description of the invention.
Lignins in the presence of a solution containing sodium hydroxide, at an elevated temperature, will depolymerize, disperse and dissolve lignins to form a black liquor. The term depolymerized lignins is often applied to lignins in a solution containing sodium hydroxide to depolymerize, disperse and dissolve lignins. Lignins are not well structured polymers. When a lignin polymer is fractured the term xe2x80x9cdepolymerized ligninsxe2x80x9d is often utilized even though no definite xe2x80x9clignin monomerxe2x80x9d exists. The term xe2x80x9cdepolymerized ligninxe2x80x9d is used interchangeably to designate lignins.
This invention relates to a black liquor or any mixture of sodium hydroxide containing depolymerized lignins, dispersed and dissolved in a solution containing sodium hydroxide. The mixture of sodium hydroxide containing depolymerized lignins dispersed and dissolved in a solution containing sodium hydroxide is generally separated by a membrane to provide depolymerized lignins and a solution: containing sodium hydroxide. The membrane, for example, can be a diffusion membrane or a nano filtration membrane. After separating, the depolymerized lignins are then counter flow extracted with water to produce depolymerized lignins substantially free of water soluble sodium compounds and an extractate containing water soluble sodium compounds. The previously extracted depolymerized lignins, substantially free of water soluble sodium compounds, necessitates an admixture of an acid to the depolymerized lignins to produce sodium salts from the depolymerized lignins containing sodium compounds. Depolymerized, lignins are prone to self condensation under acidic conditions. A solution of sodium salts is divided from the depolymerized lignins to produce depolymerized lignins free of sodium and a solution of sodium salts. The solution containing sodium hydroxide and the extractate are then combined and treated in a salt splitting procedure to remove salts and form sodium hydroxide and then the solution containing sodium hydroxide is evaporated to remove water to concentrate the solution. The concentrated solution regularly contains about 40% sodium hydroxide to about 10% sodium hydroxide. Either the solution containing sodium hydroxide or the concentrated solution must have salts removed by a salt splitter to prevent salt buildup in the concentrated solution. A salt splitting operation often utilizes either a bipolar membrane or an ion exchange resin to remove the anion from the salt. As a result sodium hydroxide is created from the salt. Salt splitting is dependent on a salt splitting action. Salt splitting is frequently accomplished with an aion ion exchange resin. The concentrated solution is then skimmed of tall oil containing sodium. Tall oil resembles a soap possessing sodium. The resulting tall oil, after separation from the concentrated solution, is then reacted with an acid to form a salt solution phase and a phase of crude tall oil.
The salt solution phase is combined with the solution of sodium salts, from the acidification of the depolymerized lignins containing sodium compounds, and then subjected to salt splitting, for example a bipolar membrane, to form a solution of sodium hydroxide and a solution of an acid and a solution of un-coverted sodium salts. The solution of un-coverted sodium salts, with water removed, is then recycled to the salt splitter. Purges are performed, as necessary, to prevent buildup of undesired impurities.
The multiple-effect evaporator, previously required to concentrate black liquor, is capable of being applied to remove water from the solution containing sodium hydroxide to produce xe2x80x9cwhite liquorxe2x80x9d for recycle. A multiple-effect evaporator is often employed in concentrating a solution, by removal of water. Means to separate the depolymerized lignins will depend on depolymerized lignins molecular properties including molecular size, density and diffusion rate. Membranes can be selected from the group of membranes including diffusion membranes and nano filtration membranes. Several other means of separation depend on depolymerized lignins molecular properties include cooling, adsorption, absorption, retardation, filtration and diffusion.
For supplementary particulars on lignin, for example, refer to Pulp And Paper Technology pages 33-36.
For information on tall oil, for example, refer to KIRK-OTHMER ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY FIFTH EDITION VOLUME 23, pages 615-622. Membranes, providing a means for separation, may employ a diffusion membrane, an ultra filtration membrane or a nano filtration membrane such as described, for example, in the bulletins supplied by membrane manufacturers Aqualytics and Desal membrane products, Inc. respectively.
Salt splitting, for example, is described in bulletins provided by several companies including Aqualytics and Dow Chemical Company.
The present invention, in its broadest aspect, is a method to yield depolymerized lignins separated from a solution containing sodium hydroxide. This invention defines a method to produce a lignin, substantially free of sodium compounds, and a solution made suitable for reuse to depolymerize, disperse and dissolve additional lignins. The method, depicted herein, originates from a mixture of depolymerized lignins dispersed and dissolved in a solution containing sodium hydroxide. The mixture containing depolymerized lignins and sodium hydroxide is substantially separated from the depolymerized lignins to supply a solution containing sodium hydroxide separated from the depolymerized lignins and will provide depolymerized lignins for extraction. The depolymerized lignins are then counter flow extracted with water to substantially extract water soluble sodium compounds from the depolymerized lignins and establish an extractate containing water soluble sodium compounds. The extracted depolymerized lignins normally contains sodium compounds. Addition of an acid to the depolymerized lignins will produce water soluble sodium salts and free the sodium contained in the depolymerized lignins. The neutralized depolymerized lignins forms a phase of a solution containing sodium salts and an insoluble phase of depolymerized lignins. Separation of the salt phase will provide depolymerized lignins substantially free of sodium salts. The salt phase is then converted to a solution of sodium hydroxide by means of a salt splitting technique.
Key features of this invention are:
Separating depolymerized lignins from sodium compounds.
The solution containing sodium hydroxide is subject to recycle for employment to depolymerize, disperse and dissolve additional lignins.
Production of depolymerized lignins, substantially free of sodium, is suitable to fire boilers to avoid explosions due to the presence of sodium.
Recycle of substantially all of the sodium compounds as sodium hydroxide and sodium sulfide.
The preferred embodiment of this invention reveals a method to produce depolymerized lignins substantially free from sodium compounds. A mixture of depolymerized lignins dispersed and dissolved in a solution containing sodium hydroxide is provided from wood. The mixture is separated to from depolymerized lignins and a liquid containing sodium hydroxide. The separated depolymerized lignins are then extracted by water to produce an extractate and a rafinate of depolymerized lignins. Sodium compounds contained in the rafinate of depolymerized lignins will be formed into water soluble salts by addition of an acid. Separation of the water soluble salts from the depolymerized lignins will provide depolymerized lignins substantially free of sodium. The solution previously separated will be used to depolymerized additional lignins. As a consequence, a mixture containing depolymerized lignins is achieved.
The features that are considered characteristic of this invention are set forth in the appended claims. This invention, however, both as to its origination and method of operations as well as additional advantages will best be understood from the following description when read in conjunction with the accompanying drawings in which:
FIG. 1 is a flow sheet denoting the invention as set forth in the appended claims.
FIG. 2 is a flow sheet denoting a method to separate tall oil from a concentrated solution containing sodium hydroxide.
FIG. 3 is a flow sheet denoting a method for forming crude tall oil.
FIG. 4 is a flow sheet denoting a method for bipolar membrane salt splitting.
FIG. 5 is a flow sheet denoting an anion exchange method for salt splitting.
FIG. 6 is a flow sheet denoting a universal method for salt splitting a solution containing salts.